Revert of Don't set cpu architecture field on iOS in UMA logs. (patchset #2 id:20001 of https://codereview.chromium.org/2671433002/ )

Reason for revert:
This breaks Chrome iOS dashboards because cpu_architecture is the only field where we have the device model info (e.g. iPhone7,2).

Original issue's description:
> Don't set cpu architecture field on iOS in UMA logs.
>
> The value we were setting it to before was not correct, so this
> change instead just makes the value not get set on iOS.
>
> BUG=370104
>
> Review-Url: https://codereview.chromium.org/2671433002
> Cr-Commit-Position: refs/heads/master@{#447578}
> Committed: https://chromium.googlesource.com/chromium/src/+/a86e8e115054ad167458d0379980db5473cb20de

TBR=rkaplow@chromium.org,asvitkine@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=370104, 718080

Review-Url: https://codereview.chromium.org/2855943005
Cr-Commit-Position: refs/heads/master@{#469062}
2 files changed